As is known, free-standing nanostructures are currently being extensively explored. These nanostructures are especially attractive for sensor applications because their high surface-to-volume ratio makes them very sensitive to charged species adsorbed on their surfaces. Lateral growth of carbon nanotubes between two electrodes during chemical vapour deposition (CVD) has been realized for which a lateral electric field applied between the electrodes attracts the growing nanotube towards the counter-electrode. Although the potential of carbon nanotubes for applications such as gas-sensing field-effect devices has been demonstrated, the difficulty of synthesizing only metallic or only semiconducting nanotubes and the difficulty of modifying nanotube surfaces have limited their development as nanosensors.
Lateral growth of GaAs nanowhiskers by metalorganic vapour phase epitaxy (MOVPE) from vertical ridges on a GaAs substrate using Au nanoparticles as the catalyst has also been demonstrated. However, the properties of the mechanical connection between a bridging nanowire and the vertical sidewall have not been favorable.
Nanowires made of silicon are especially attractive because of silicon's compatibility with existing IC processes. Moreover, the chemical and physical properties of silicon can be controlled to adjust the device sensitivity, and silicon nanowires can be selectively grown. Using silicon allows the vast knowledge of silicon technology to be applied to applications such as sensing. Using semiconductor nanowires, researchers have demonstrated electrical sensors for biological and chemical species, and designed a range of nanoelectronic and photonic devices in different material systems. In many of these demonstrations, nanowires were assembled after growth into parallel or crossed arrays by alignment aided by fluid flow or by applying electric fields. In other cases, electrical contacts were defined with electron-beam lithography on a few selected nanowires.
Although connecting electrodes to nanowires one at a time contributes to understanding the characteristics of nanowires and exploring novel device applications, it cannot be used for reproducible mass-fabrication of dense, low-cost device arrays. A massively parallel, self-assembly technique is needed to allow bridging of silicon nanowires between electrodes using only relatively coarse lithography principles.
It would, therefore, be desirable to overcome the aforesaid and other disadvantages.